tests: interface: implement toy electrum server

diff --git a/electrum/interface.py b/electrum/interface.py
index c9f438e..2411a06 100644
--- a/electrum/interface.py
+++ b/electrum/interface.py
@@ -529,6 +529,7 @@ class Interface(Logger):
         assert isinstance(server, ServerAddr), f"expected ServerAddr, got {type(server)}"
         self.ready = network.asyncio_loop.create_future()
         self.got_disconnected = asyncio.Event()
+        self._blockchain_updated = asyncio.Event()
         self.server = server
         Logger.__init__(self)
         assert network.config.path
@@ -1058,6 +1059,8 @@ class Interface(Logger):
                 self.logger.info(f"new chain tip. {height=}")
             if blockchain_updated:
                 util.trigger_callback('blockchain_updated')
+                self._blockchain_updated.set()
+                self._blockchain_updated.clear()
             util.trigger_callback('network_updated')
             await self.network.switch_unwanted_fork_interface()
             await self.network.switch_lagging_interface()
@@ -1100,6 +1103,8 @@ class Interface(Logger):
                     continue
                 # report progress to gui/etc
                 util.trigger_callback('blockchain_updated')
+                self._blockchain_updated.set()
+                self._blockchain_updated.clear()
                 util.trigger_callback('network_updated')
                 height += num_headers
                 assert height <= next_height+1, (height, self.tip)
diff --git a/tests/__init__.py b/tests/__init__.py
index 624a3e3..51decf3 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -28,6 +28,7 @@ class ElectrumTestCase(unittest.IsolatedAsyncioTestCase, Logger):
     """Base class for our unit tests."""
 
     TESTNET = False
+    REGTEST = False
     TEST_ANCHOR_CHANNELS = False
     # maxDiff = None  # for debugging
 
@@ -41,19 +42,22 @@ class ElectrumTestCase(unittest.IsolatedAsyncioTestCase, Logger):
     @classmethod
     def setUpClass(cls):
         super().setUpClass()
-        if cls.TESTNET:
+        assert not (cls.REGTEST and cls.TESTNET), "regtest and testnet are mutually exclusive"
+        if cls.REGTEST:
+            constants.BitcoinRegtest.set_as_network()
+        elif cls.TESTNET:
             constants.BitcoinTestnet.set_as_network()
 
     @classmethod
     def tearDownClass(cls):
         super().tearDownClass()
-        if cls.TESTNET:
+        if cls.TESTNET or cls.REGTEST:
             constants.BitcoinMainnet.set_as_network()
 
     def setUp(self):
         self._test_lock.acquire()
         super().setUp()
-        self.electrum_path = tempfile.mkdtemp()
+        self.electrum_path = tempfile.mkdtemp(prefix="electrum-unittest-base-")
         assert util._asyncio_event_loop is None, "global event loop already set?!"
 
     async def asyncSetUp(self):
diff --git a/tests/test_interface.py b/tests/test_interface.py
index 84e223e..e84026b 100644
--- a/tests/test_interface.py
+++ b/tests/test_interface.py
@@ -1,4 +1,12 @@
-from electrum.interface import ServerAddr
+import asyncio
+
+import aiorpcx
+
+from electrum.interface import ServerAddr, Interface, PaddedRSTransport
+from electrum import util, blockchain
+from electrum.util import OldTaskGroup, bfh
+from electrum.logging import Logger
+from electrum.simple_config import SimpleConfig
 
 from . import ElectrumTestCase
 
@@ -46,3 +54,134 @@ class TestServerAddr(ElectrumTestCase):
                          ServerAddr(host="2400:6180:0:d1::86b:e001", port=50002, protocol="s").to_friendly_name())
         self.assertEqual("[2400:6180:0:d1::86b:e001]:50001:t",
                          ServerAddr(host="2400:6180:0:d1::86b:e001", port=50001, protocol="t").to_friendly_name())
+
+
+class MockNetwork:
+
+    def __init__(self, *, config: SimpleConfig):
+        self.config = config
+        self.asyncio_loop = util.get_asyncio_loop()
+        self.taskgroup = OldTaskGroup()
+        blockchain.read_blockchains(self.config)
+        blockchain.init_headers_file_for_best_chain()
+        self.proxy = None
+        self.debug = True
+        self.bhi_lock = asyncio.Lock()
+
+    async def connection_down(self, interface: Interface):
+        pass
+    def get_network_timeout_seconds(self, request_type) -> int:
+        return 10
+    def check_interface_against_healthy_spread_of_connected_servers(self, iface_to_check: Interface) -> bool:
+        return True
+    def update_fee_estimates(self, *, fee_est: dict[int, int] = None):
+        pass
+    async def switch_unwanted_fork_interface(self):
+        pass
+    async def switch_lagging_interface(self):
+        pass
+
+
+# regtest chain:
+BLOCK_HEADERS = {
+    0: bfh("0100000000000000000000000000000000000000000000000000000000000000000000003ba3edfd7a7b12b27ac72c3e67768f617fc81bc3888a51323a9fb8aa4b1e5e4adae5494dffff7f2002000000"),
+    1: bfh("0000002006226e46111a0b59caaf126043eb5bbf28c34f3a5e332a1fc7b2b73cf188910f186c8dfd970a4545f79916bc1d75c9d00432f57c89209bf3bb115b7612848f509c25f45bffff7f2000000000"),
+    2: bfh("00000020686bdfc6a3db73d5d93e8c9663a720a26ecb1ef20eb05af11b36cdbc57c19f7ebf2cbf153013a1c54abaf70e95198fcef2f3059cc6b4d0f7e876808e7d24d11cc825f45bffff7f2000000000"),
+    3: bfh("00000020122baa14f3ef54985ae546d1611559e3f487bd2a0f46e8dbb52fbacc9e237972e71019d7feecd9b8596eca9a67032c5f4641b23b5d731dc393e37de7f9c2f299e725f45bffff7f2000000000"),
+    4: bfh("00000020f8016f7ef3a17d557afe05d4ea7ab6bde1b2247b7643896c1b63d43a1598b747a3586da94c71753f27c075f57f44faf913c31177a0957bbda42e7699e3a2141aed25f45bffff7f2001000000"),
+    5: bfh("000000201d589c6643c1d121d73b0573e5ee58ab575b8fdf16d507e7e915c5fbfbbfd05e7aee1d692d1615c3bdf52c291032144ce9e3b258a473c17c745047f3431ff8e2ee25f45bffff7f2000000000"),
+    6: bfh("00000020b833ed46eea01d4c980f59feee44a66aa1162748b6801029565d1466790c405c3a141ce635cbb1cd2b3a4fcdd0a3380517845ba41736c82a79cab535d31128066526f45bffff7f2001000000"),
+    7: bfh("00000020abe8e119d1877c9dc0dc502d1a253fb9a67967c57732d2f71ee0280e8381ff0a9690c2fe7c1a4450c74dc908fe94dd96c3b0637d51475e9e06a78e944a0c7fe28126f45bffff7f2000000000"),
+    8: bfh("000000202ce41d94eb70e1518bc1f72523f84a903f9705d967481e324876e1f8cf4d3452148be228a4c3f2061bafe7efdfc4a8d5a94759464b9b5c619994d45dfcaf49e1a126f45bffff7f2000000000"),
+    9: bfh("00000020552755b6c59f3d51e361d16281842a4e166007799665b5daed86a063dd89857415681cb2d00ff889193f6a68a93f5096aeb2d84ca0af6185a462555822552221a626f45bffff7f2000000000"),
+    10: bfh("00000020a13a491cbefc93cd1bb1938f19957e22a134faf14c7dee951c45533e2c750f239dc087fc977b06c24a69c682d1afd1020e6dc1f087571ccec66310a786e1548fab26f45bffff7f2000000000"),
+    11: bfh("00000020dbf3a9b55dfefbaf8b6e43a89cf833fa2e208bbc0c1c5d76c0d71b9e4a65337803b243756c25053253aeda309604363460a3911015929e68705bd89dff6fe064b026f45bffff7f2002000000"),
+    12: bfh("000000203d0932b3b0c78eccb39a595a28ae4a7c966388648d7783fd1305ec8d40d4fe5fd67cb902a7d807cee7676cb543feec3e053aa824d5dfb528d5b94f9760313d9db726f45bffff7f2001000000"),
+}
+
+_active_server_sessions = set()
+def _get_active_server_session() -> 'ServerSession':
+    assert 1 == len(_active_server_sessions), len(_active_server_sessions)
+    return list(_active_server_sessions)[0]
+
+class ServerSession(aiorpcx.RPCSession, Logger):
+
+    def __init__(self, *args, **kwargs):
+        aiorpcx.RPCSession.__init__(self, *args, **kwargs)
+        Logger.__init__(self)
+        self.logger.debug(f'connection from {self.remote_address()}')
+        self.cur_height = 6  # type: int  # chain tip
+        _active_server_sessions.add(self)
+
+    async def connection_lost(self):
+        await super().connection_lost()
+        self.logger.debug(f'{self.remote_address()} disconnected')
+        _active_server_sessions.discard(self)
+
+    async def handle_request(self, request):
+        handlers = {
+            'server.version': self._handle_server_version,
+            'blockchain.estimatefee': self._handle_estimatefee,
+            'blockchain.headers.subscribe': self._handle_headers_subscribe,
+            'blockchain.block.header': self._handle_block_header,
+            'blockchain.block.headers': self._handle_block_headers,
+            'server.ping': self._handle_ping,
+        }
+        handler = handlers.get(request.method)
+        coro = aiorpcx.handler_invocation(handler, request)()
+        return await coro
+
+    async def _handle_server_version(self, client_name='', protocol_version=None):
+        return ['best_server_impl/0.1', '1.4']
+
+    async def _handle_estimatefee(self, number, mode=None):
+        return 1000
+
+    async def _handle_headers_subscribe(self):
+        return {'hex': BLOCK_HEADERS[self.cur_height].hex(), 'height': self.cur_height}
+
+    async def _handle_block_header(self, height):
+        return BLOCK_HEADERS[height].hex()
+
+    async def _handle_block_headers(self, start_height, count):
+        assert start_height <= self.cur_height, (start_height, self.cur_height)
+        last_height = min(start_height+count-1, self.cur_height)  # [start_height, last_height]
+        count = last_height - start_height + 1
+        headers = b"".join(BLOCK_HEADERS[idx] for idx in range(start_height, last_height+1))
+        return {'hex': headers.hex(), 'count': count, 'max': 2016}
+
+    async def _handle_ping(self):
+        return None
+
+
+class TestInterface(ElectrumTestCase):
+    REGTEST = True
+
+    def setUp(self):
+        super().setUp()
+        self.config = SimpleConfig({'electrum_path': self.electrum_path})
+        self._orig_WAIT_FOR_BUFFER_GROWTH_SECONDS = PaddedRSTransport.WAIT_FOR_BUFFER_GROWTH_SECONDS
+        PaddedRSTransport.WAIT_FOR_BUFFER_GROWTH_SECONDS = 0
+
+    def tearDown(self):
+        PaddedRSTransport.WAIT_FOR_BUFFER_GROWTH_SECONDS = self._orig_WAIT_FOR_BUFFER_GROWTH_SECONDS
+        super().tearDown()
+
+    async def asyncSetUp(self):
+        await super().asyncSetUp()
+        self._server: asyncio.base_events.Server = await aiorpcx.serve_rs(ServerSession, "127.0.0.1")
+        server_socket_addr = self._server.sockets[0].getsockname()
+        self._server_port = server_socket_addr[1]
+        self.network = MockNetwork(config=self.config)
+
+    async def asyncTearDown(self):
+        self._server.close()
+        await super().asyncTearDown()
+
+    async def test_client_syncs_headers_to_tip(self):
+        interface = Interface(network=self.network, server=ServerAddr(host="127.0.0.1", port=self._server_port, protocol="t"))
+        self.network.interface = interface
+        await util.wait_for2(interface.ready, 5)
+        await interface._blockchain_updated.wait()
+        self.assertEqual(_get_active_server_session().cur_height, interface.tip)
+        self.assertFalse(interface.got_disconnected.is_set())